1. Field of the Invention
This invention relates to fluorescent lamps containing a mixture of phosphors having different particle sizes.
2. Description of the Prior Art
Fluorescent lamps have been used as a general source of illumination light for many years.
In order to obtain a given desired color rendition using fluorescent lamps with a high light output, it has been proposed to blend different luminescent materials with one another or to apply them in superposed layers. For example, U.S. Pat. No. 4,088,923 describes a fluorescent lamp having two luminescent layers. In particular, the luminescent material in the layer (i.e. the first layer) more remote from the discharge is cheaper than that in other layer (i.e. the second layer). The first layer is composed of well known calciumhalophoshate phosphor. The second layer is composed of a mixture of three phosphors, i.e. blue emitting phosphor, green emitting phosphor and red emitting phosphor. The desired mixture of wave lengths is achieving by mixing the three phosphors in the proper ratio. When manufacturing fluorescent lamps on a large scale using such phosphors, there occurs the problem of uneven luminescence in the individual fluorescent lamp produced. Furthermore, there is variation in the luminescent properties from one lamp to the next in a product run.
Accordingly, a need exists for fluorescent lamps having more uniform luminescent properties.